Methods and apparatus for sensing ground leakage and automated self testing thereof

ABSTRACT

Methods and apparatus provide for: measuring leakage current from a source of AC power, which provides power to at least one load circuit; and comparing the measured leakage current values against one or more predetermined thresholds to establish status therefor; and automatically performing self tests to determine whether the leakage current sensing and comparing operations are operative.

BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for sensingleakage current in a system, and more particularly, to a circuit that iscapable of sensing such leakage and also operates to performself-testing to ensure proper sensing capability.

In alternating current (AC) power circuits, current from a single phaseAC source normally flows from an AC source via a phase wire, through aload that is fed by the AC power, back through a neutral wire to the ACsource, and vise verse. Any component of current that does not flow tothe load via the phase wire and from the load via the neutral wire willflow via one or more leakage paths to earth ground. Such current isknown as leakage or residual current. Leakage current is dangerousbecause it can lead to electrocution and/or fires if not properlycontrolled.

Ground Fault Interrupters (GFIs), which are also known as ResidualCurrent Detectors (RCDs), are commonly available devices for detectingand interrupting leakage current flow. A diagram of a typical GFIcircuit 10 is shown in FIG. 1. By way of example, a single phase ACsource of power is provided by way of a phase wire L and a neutral wireN. The phase and neutral wires L, N are coupled to the load 14 (which ina residence may be an appliance of some kind), and AC current cyclesfrom the source over the phase wire L, through the load 14, and back tothe source via the neutral wire N, and vise verse.

The GFI circuit 10 includes a differential current transformer (T1),including a toroidal core through which the phase and neutral wires L, Npass. By passing through the current transformer, T1, the phase andneutral wires L, N function as the primary winding of T1. The secondarywinding 12 includes a number of turns of wire wound around the core ofthe transformer T1. The secondary winding 12 is coupled to a circuitinterrupter 16. The circuit interrupter 16 includes a control function16A and a switch 16B, which is illustrated as a double pole, singlethrow (DPST) switch. The control function 16A operates to actuate (open)the DPST switch 16B when there is voltage present across, and/or currentthrough, the secondary winding 12.

When there is no leakage current to earth ground, the sum of therespective AC currents in the phase and neutral wires L, N (through theprimary winding) of the transformer T1 is zero. This results a net zerocurrent flow in the primary of the transformer T1, which in turn resultsin no induced current in the secondary winding 12. Thus, the controlfunction 16A does not open the DPST switch 16B and the AC currentcontinues to flow through the load 14 from the AC source.

When there is leakage current to ground, however, the sum of therespective AC currents in the phase and neutral wires L, N (through theprimary winding) of the transformer T1 is not zero. Indeed, for example,if some current were to flow from the AC source over the phase wire L,through earth ground, and back to the AC source, effectively bypassingthe transformer T1, then there would be a greater magnitude AC currentflowing over the phase wire L, through the primary of the transformerT1, than would return over the neutral wire N, through the primary ofthe transformer T1. The imbalance in current through the primary windingof the transformer T1 caused by the leakage current flow induces acurrent in the secondary winding 12 of the transformer T1. The inducedcurrent in the secondary winding 12 is sensed by the control function16A and opens the DPST switch 16B, thereby interrupting the leakagecurrent as well as the power to the load 14. In a typical GFI circuit10, once the circuit interrupter 16 trips (the DPST switch 16B opens),the leakage path must be cleared, and a user must manually reset theswitch 18B to the closed state.

In order to permit a user to verify that the GFI circuit 10 isoperational, i.e., that an apparent zero primary winding current and/orzero secondary winding current is not due to a malfunction, the GFIcircuit 10 includes a test feature. The test feature is implemented viaa test button or switch S1, which may be manually pressed by the user.Pressing the test switch S1 is intended to cause the circuit interrupter16 to trip and open the DPST switch 16B, thereby indicating that the GFIcircuit 10 operates properly. Pressing the test switch S1 causes a smallresistive load R1 to draw a current that bypasses the primary winding ofthe transformer T1. The bypass current has the same effect as leakagecurrent in that the sum of the respective AC currents through the phaseand neutral wires L, N, passing through the primary winding of thetransformer T1 is non-zero. The non-zero current through the primarywinding induces a current in the secondary winding 12 of the transformerT1, which is sensed by the control function 16A and opens the DPSTswitch 16B, thereby interrupting current paths over the phase andneutral wires L, N.

While leakage current is of concern in Information Technology (IT)equipment rooms, the conventional GFI circuit 10 is not a suitablesolution to leakage current problems. IT equipment rooms (also known asdata centers) utilize hundreds or even thousands of units of ITequipment. Each piece of IT equipment receives primary AC power byplugging into an outlet of a power distribution unit (PDU). A PDU isalso a piece of IT equipment and typically includes: (a) a high powerinlet from which power is received (typically from a panel board); (b)multiple lower power outlets; and (c) (optional) circuit breakers orfuses to protect the outlets from over current conditions (shortcircuits, etc.). PDUs are often designed to report certain statusinformation over a communication and/or input/output interface,including: (a) the voltage being supplied to a given PDU's inlet, (b)how much power is flowing in the inlet and each outlet, and (c) the tripstate (whether voltage is present) of each circuit breaker.

In a data center, it is not practical to use a standard GFI circuit 10for a number of reasons. For example, it would be far too disruptive tounconditionally interrupt AC power to an IT device due to leakagecurrent. Indeed, sensitive data may be corrupted and/or irrevocably lostif AC power were interrupted without notice. Additionally, industrialequipment (such as in data centers) may not be permitted to exhibit thesame level of leakage current as those established in conventionalresidential GFI circuits 10. Indeed, the permissible level of leakagecurrent in a data center (and/or other industrial environment) may be upto about 3.5 mA of current. Standard GFI circuits, however, may employ afixed trip threshold for leakage current that is not appropriate for adata center. Still further, since data centers include thousands ofunits of IT equipment, it would be far too time-consuming, andsusceptible to error, to manually test each GFI circuit 10.

Although the prior art systems address some issues associated withleakage current, the known solutions are unsatisfactory in the contextof a data center (or other industrial environment). There are,therefore, needs in the art for new methods and apparatus for sensingground leakage current in a system, and more particularly, for sensingsuch ground leakage and also operating to perform self-testing to ensureproper sensing capability.

SUMMARY OF THE INVENTION

In accordance with one or more aspects of the present inventiondescribed herein, a system for sensing leakage current includes one ormore of the following features: (i) the ability to automatically, andpreferably on a programmable, periodic basis, perform a self-test of thecritical system hardware and/or software used to measure and monitorleakage current; (ii) the ability to permit an operator to set one ormore leakage current levels, such as a warning level and/or a criticallevel; (iii) the ability to visually display a status indication of oneor more leakage current conditions, such as an actual RMS leakagecurrent measurement, an indication of a normal (or acceptable) leakagecurrent level, an indication of a warning (e.g., elevated but notcritical) leakage current level, an indication of a critical (e.g., anunacceptably, or near unacceptably, elevated) leakage current level, anindication that a self-test has uncovered faulty hardware and/orsoftware; and (iv) the ability to dispatch notifications reporting thepresence of one or more of the aforementioned status indications (and/ora change in such status), such as by way of the simple networkmanagement protocol (SNMP), e-mail, and/or other electronic means.

Other aspects, features, and advantages of the present invention will beapparent to one skilled in the art from the description herein taken inconjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

For the purposes of illustration, there are forms shown in the drawingsthat are presently preferred, it being understood, however, that theinvention is not limited to the precise arrangements andinstrumentalities shown.

FIG. 1 is a block diagram of a conventional GFI circuit for sensing andinterrupting AC power to a load when leakage current is present inaccordance with the prior art;

FIG. 2 is a block diagram of a leakage current sensing system inaccordance with one or more aspects of the present invention;

FIG. 3 is a generalized representation of a visual user-interfacesuitable for use in the leakage current sensing system of FIG. 2 and/orother embodiments disclosed and/or described herein;

FIG. 4 is a diagram of a process flow that may be employed in theleakage current sensing system of FIG. 2 and/or other embodimentsdisclosed and/or described herein; and

FIG. 5 is a more detailed schematic diagram of a circuit that issuitable for implementing the leakage current sensing system of FIG. 2and/or other embodiments disclosed and/or described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings wherein like numerals indicate likeelements there is shown in FIG. 2 a block diagram of a system 100 inaccordance with one or more aspects of the present invention. The system100 includes a leakage current sensor 102, and a control circuit 104.The leakage current sensor 102 operates to measure leakage current froma source of AC power, which provides power to at least one load circuit140. In accordance with one or more aspects of the present invention, itis specifically contemplated that the load circuit 140 includes one ormore pieces of IT equipment in a data center.

The control circuit 104 operates to take certain action in response toreceived measured leakage current values from the leakage current sensor102. The primary actions that the control circuit 104 circuit takes withrespect to the received measured leakage current values are: (i) tocompare the measured leakage current values against one or morepredetermined thresholds to establish status therefor; and (ii) toperform one or more self tests to determine whether the leakage currentsensor 102 is operative.

With regard to the former action, comparing the measured leakage currentvalues against one or more predetermined thresholds, the operation ofthe system 100 is as follows. The AC source of power is coupled to theload 140 by way of a phase wire L and a neutral wire N. The leakagecurrent sensor 102 includes a differential current transformer (T100),which includes a core, e.g., a toroidal core, through which the phaseand neutral wires L, N pass. By passing through the center of thetoroidal core of the transformer T100, the phase and neutral wires L, Nestablish a bifilar, single turn coil, which acts as a primary windingof T100. The secondary winding 120 includes a number of turns of wirewound around the core of the transformer T100. The secondary winding 120is input into a signal conditioning circuit 106, such ascurrent-to-voltage converter circuit, which produces a voltageindicative of the current induced into the secondary winding 120 of thetransformer T100.

It is noted that, for purposes of discussion, the AC source of power isassumed to be a single phase source, where the legs of the AC source arethe phase and neutral wires. It is understood, however, that theembodiments of the invention disclosed and described herein may bereadily extended to other types of AC sources, including a split phaseconfiguration, and/or a three phase source. In a split phaseconfiguration, the legs are respective phase wires of a multi-phase ACsource, such as line-1 and line-2; line-1 and neutral; or line-2 andneutral. In a three phase AC source, the legs may be line-1, line-2, andline-3; or line-1, line-2, line-3, and neutral. Indeed, for any of theother type of AC sources, the legs simply pass through the center of thetoroidal core of the transformer T100, such that the legs form amultifilar, single turn coil, which acts as the primary winding of T100.Thus, while the embodiments illustrated in the figures and discussedherein are in the context of a single phase AC source, it is understoodthat any of such embodiments may readily be extended to operate withother types of AC sources.

The voltage induced into the secondary winding 120 of the transformerT100 is an indicator of the leakage current in the system 100. Asdiscussed earlier in this description, without any leakage current,precisely the same magnitude of current flows in each of the phase andneutral wires L, N; however, each such magnitude of current is ofopposite polarity flowing through the core. If leakage current ispresent, then such equal but opposite current flow does not exist withrespect to the core, and some amount of current finds its way back tothe AC source through another path, e.g., through earth ground, EG. Whenthere is no leakage current, there is no net current flow in the primarywinding of the transformer T100, which in turn results in no inducedcurrent in the secondary winding 120. The voltage produced by the signalconditioning circuit 106 is therefore substantially zero (although theremay be some noise component in a practical circuit). When there isleakage current, however, the sum of the respective AC currents in thephase and neutral wires L, N through the core is not zero. The imbalancein current through the primary winding of the transformer T100 caused bythe leakage current flow induces a current in the secondary winding 120of the transformer T1. The voltage produced by the signal conditioningcircuit 106 is therefore some analog value indicative of the current inthe secondary winding 120.

The control circuit 104 is preferably implemented by way of a suitablemicrocontroller, which operates to execute software/firmwareinstructions in order to achieve desirable operation of the system 100.Such a microcontroller 104 may include one or more high impedance inputpins (labeled INPUT in FIG. 2) for receiving analog voltage signals,such as the voltage from the signal conditioning circuit 106. Moreparticularly, the microcontroller 104 preferably includes at least oneinput pin that is set up to receive an analog voltage signal, andconvert same to a digital signal for manipulation. In an alternativeimplementation, if the microcontroller 104 does not include suitableinternal circuitry, an external analog-to-digital (A/D) converter (notshown) may be employed between the signal conditioning circuit 106 and adigital input pin of the microcontroller 104.

The microcontroller 104 may be implemented utilizing any of the knowntechnologies, such as commercially-available microprocessors, digitalsignal processors, any of the known processors that are operable toexecute software and/or firmware programs, programmable digital devicesor systems, programmable array logic devices, or any combination of theabove, including devices now available and/or devices which arehereinafter developed. By way of example, the microcontroller 104 may beimplemented using the STM32 ARM MCU, which is available from a companycalled STMicroelectronics.

Software/firmware being executed by the microcontroller 104 is operableto mathematically manipulate the data presented at the INPUT terminal,which is a digitized version of the measured current induced into thesecondary winding 120 of the transformer T100. Among such mathematicalmanipulations is preferably the aforementioned comparison of themeasured leakage current values against one or more predeterminedthresholds. In this regard, the microcontroller 104 operates to convertthe digital values representing the current in the secondary winding 120of the transformer T100 into an actual RMS value of the leakage current.This conversion formula will be well understood to a skilled artisanfrom the description herein. The conversion will be a function of thenumber of turns of the primary winding of the transformer T100, thenumber of turns of the secondary winding 120 of the transformer T100,and the transform characteristics of the signal conditioning circuit106.

In accordance with one or more aspects of the invention, and asillustrated in FIGS. 2 and 3, the system 100 may include a display 108operatively coupled to the microcontroller 104, wherein the display 108includes some type of screen 300 providing a visual indication of themeasured leakage current values 302. Advantageously, an operator withinthe data center may easily obtain an indication as to the leakagecurrent in the one or more pieces of IT equipment. It is furthercontemplated that a plurality of separate leakage current sensors 102and/or microcontrollers 104 may be employed to separately measureleakage current in (and/or other functions discussed later herein)individual pieces of IT equipment or respective groups of suchequipment.

Returning to the specific action of comparing the measured leakagecurrent values against one or more predetermined thresholds, themicrocontroller 104 operates to execute such function by way of thesoftware/firmware running thereon. The system 100 may comepre-programmed with the one or more predetermined threshold levels.Preferably, however, the microcontroller 104 is programmable by theoperator in order to set at least one of the one or more predeterminedthresholds. In this regard, reference is made to FIGS. 3 and 4, thelatter being a flow chart illustrating certain process steps and/orfunctions 400 of the system 100. In general, the process 400 isindicative of at least certain portions of the software/firmware runningon the microcontroller 104 to achieve desirable operation of the system100.

At action 402 of FIG. 4, the operator may be prompted to enter, or mayproactively induce the microcontroller 104 to receive, certain settings,such as one or more predetermined thresholds. In this regard, if theoperator is not permitted to set the one or more thresholds, the processflow branches in the negative to action 406. If, however, the operatoris permitted to set the one or more thresholds, the process flowbranches in the affirmative to action 404. By way of example, theoperator may set a first threshold level indicating a leakage current,which if present would be higher than desired and/or otherwise excessivewhen considering the specifications of the particular application. Thefirst threshold level may be considered a warning level because, eventhough the level is elevated, the system may continue to operate, thethreat posed by the level may not be extreme, etc. By way of furtherexample, the operator may set a second threshold level indicating aleakage current, which if present would be higher than the firstthreshold level, substantially higher than desired, and excessive whenconsidering the normal operation of the particular system undermeasurement. The second threshold level may be considered a criticallevel because the system may not operate properly, the threat posed bythe level may be unacceptable, etc.

At action 404, the settings for the first and second threshold levelsare stored in memory (not shown) and used during the comparison process(as will be discussed below). In addition, as shown in FIG. 3, the firstand second threshold levels may be displayed at location 304 of thedisplay 108 as a warning level and a critical level, respectively.Notably, the levels listed at location 304 of the display 108 mayindicate a dual function, both displaying the selected levels andoperating as inputs of the operator's selection to the microcontroller104, in which case the display 108 operates as an input/output device.Once action 404 is complete, the process flow continues to action 406.

At action 406, the microcontroller 104 reads the leakage current value(e.g., obtains a digital representation of the current in the secondarywinding 120 of the transformer T100 and converts same into a value ofmeasured leakage current, preferably an RMS value). Next, at action 408,the microcontroller 104 determines whether the measured value of leakagecurrent is above the critical value. If negative, the process flowbranches to action 412, where the microcontroller 104 determines whetherthe measured value of leakage current is above the warning level. Anaffirmative determination at action 408, or an affirmative or negativedetermination at action 412 will result in some conclusive statuscondition.

An affirmative determination at action 408 causes the process flow tobranch to action 410 and indicates that the measured leakage current inthe system is above the second threshold level, the critical level,which in this example is above 10 mA. At action 410 the microcontroller104 sets the status level to critical and potentially takes furtheraction. For example, upon a status of critical (and/or upon a change instatus from something else to critical), the microcontroller 104 mayoperate to display the status on the display 108, such as at location306, where the leakage status level, labeled “Critical”, is highlighted.Alternatively or additionally, the microcontroller 104 may operate tostore such status in an event log. The status may include the level(critical), the specific current magnitude, the threshold setting, thetime, the date, the duration of the condition, etc. Thereafter, anoperator and/or another program may access the event log via themicrocontroller 104 in order to obtain the status data, consider same,and validate the ability of the system 100 to monitor the leakagecurrent.

Additionally or alternatively, the microcontroller 104 may operate tosend a notification of the status (or change in status) to one or moredestinations. In this regard, the system 100 may include a messagetransmission system 110 coupled to the microcontroller 104 (and/orintegral thereto), which operates to transmit one or more messagesindicating the aforementioned status. Those skilled in the art willappreciate that the one or more messages may be of any number ofspecific protocol(s), such as: (i) a simple network management protocol,(ii) a packet switched network protocol, (iii) an electronic mailprotocol, (iv) an instant messaging protocol, (v) a telephone protocol,and/or any other suitable protocol. Preferably, the message transmissionsystem 110 is programmable by the operator to select the messagingprotocol, and the display 108 shows a visual indication of the selectedmessaging protocol at location 308. Notably, the protocols listed atlocation 308 of the display 108 may provide a dual function, bothdisplaying the selected protocol(s) and operating as inputs of theoperator's selection to the microcontroller 104, in which case thedisplay 108 operates as an input/output device.

Once the processes associated with action 410 are complete, the processflow loops back to action 406, where the process there is repeated solong as the critical status exists.

An affirmative determination at action 412 causes the process flow tobranch to action 414 and indicates that the measured leakage current inthe system is above the first threshold level (but not the secondthreshold level), i.e., the warning level, which in this example isabove 4 mA but less than 10 mA. At action 414 the microcontroller 104sets the status level to warning and potentially takes further action.For example, upon a status of warning (and/or upon a change in statusfrom something else to warning), the microcontroller 104 may operate todisplay the status at location 306, by highlighting the level labeled“Warning”. Alternatively or additionally, the microcontroller 104 mayoperate to store such status in an event log, which may include one ormore of the level (warning), the specific current magnitude as measured,the threshold setting, the time, the date, the duration of thecondition, etc. Additionally or alternatively, the microcontroller 104may operate to send a notification of the status (or change in status)to one or more destinations in a manner similar to that discussed abovewith respect to the status of critical.

Once the processes associated with action 414 are complete, the processflow loops back to action 406, where the process there is repeated solong as the warning status exists.

Further, a negative determination at action 412 causes the process flowto branch to action 416 and indicates that the measured leakage currentin the system is not above the first or second threshold levels, i.e.,the normal level, which in this example is below 4 mA. At action 416 themicrocontroller 104 sets the status level to normal. Possible furtheraction includes one or more of: (i) displaying the status at location306, by highlighting the level labeled “Normal”, (ii) storing suchstatus in an event log, and (iii) sending a notification of the status(or change in status) to one or more destinations.

Once the processes associated with action 416 are complete, the processflow advances to action 418. Returning to an earlier discussion of theprimary actions that the microcontroller 104 takes with respect to thereceived measured leakage current values, the second general action ispreferably performing (preferably automatically) one or more self teststo determine whether the leakage current sensor 102 is operative. Beforediscussing the specific functions and aspects associated with action 418et seq., a discussion of the structure and operation of the system 100as concerns self testing capabilities will be presented.

Turning to FIG. 2, the system 100 includes a means for self testing theleakage current sensing capabilities of the system 100. In particular,the leakage current sensor 102 includes a switch circuit S100 that turnson and off in response to a control signal, GPIO, output from themicrocontroller 104, and a resistance R100. The switch circuit T100 andthe resistance R100 are coupled in series between the phase wire L andthe neutral wire N, such that an unbalanced flux is produced within thecore of the transformer T100 when the switch S100 turns on. In theexample shown, the series switch S100 and resistance R100 are configuredsuch that current flowing therethrough bypasses the core of thetransformer T100. Thus, the current flowing in the phase wire L and theneutral wire N will not be equal but opposite through the core, therebycreating an additional component of flux in the core that induces acurrent in the secondary winding 120 of the transformer T100. The valueof the resistance R100 is chosen such that the induced current in thesecondary winding 120 is sufficient to test the leakage current sensingcapabilities of the system 100.

The software/firmware of the microcontroller is preferably operable toestablish the proper potential on the switch circuit S100 (which may beimplemented using one or more transistors, such as a MOSFET, etc.) inorder to turn same on an off. The GPIO pin may be capable of operatingas an output, where the state of the GPIO pin may be OFF or ON,depending on the commands established by software/firmware beingexecuted on the microcontroller 104. The OFF state is defined as a logic“low” level, which may be any suitable voltage potential (often about 0volts, or ground), and in such state the GPIO pin is capable of sinkingcurrent (into a relatively low impedance). The ON state is defined as alogic “high” level, which again may be any suitable voltage potential.The actual voltage of the GPIO pin in the ON state is often dictated bythe operating DC supply voltage to the microcontroller 104. By way ofexample, such logic high voltage level may be anywhere between about0.333 to about 5 VDC (with reference to ground), although lower andhigher voltage levels are also possible. In the ON state, the GPIO pinis capable of sourcing current at the logic high voltage level (from arelatively low source impedance).

The implementation of the switch circuit S100 will establish that theswitch turns on upon an ON state of the GPIO pin and that the switchturns off upon an OFF state of the GPIO pin. It is noted that thespecific implementation of the switch circuit S100 will dictate whetherthe switch turns on in response to the GPIO in the ON state or the OFFstate, either relationship will suffice. When the switch circuit S100turns on and current is induced into the secondary winding 120, themicrocontroller 104 measures the faux leakage current and compares sameto a threshold level, which may be one of the aforementioned warningand/or critical levels, and/or may be another level specific to a selftest.

Turning again to FIGS. 3 and 4, at action 418, a determination is madeas to whether the conditions are ripe for a self test procedure. By wayof example, the conditions may include a time, date, interval, and/orother metric. In some embodiments, it is desirable to conduct the selftest periodically, such as every 24 hours. In this regard, the operatormay set the conditions for self test at action 402 (discussed above withrespect to settings), and such setting(s) may be displayed at location304. Again, if the display 108 operates as an input/output device, theoperator may set the self test conditions (in this example an intervalof 24 hours) through manipulation of the screen 300 or other inputelement of the display 108.

An affirmative determination at action 418 causes the process flow tobranch to action 420 and indicates that the status of the system 100 isin self test mode. The display 108 may indicate the self test mode atlocation 306 (FIG. 3) by highlighting the label “Testing”. As discussedabove, the software/firmware may produce a control signal on the GPIOpin of the microcontroller 104, which turns on the switch circuit S100.The microcontroller 104 then compares the measured leakage current toone or more thresholds, such as 4 mA, 10 mA, and/or other thresholds.The process then advances to action 422, where a determination is madeas to whether the result of the comparison at action 420 indicates thatthe leakage current sensing circuitry failed the self test. If theresult of the determination is negative (i.e., that the test passed),then the process flow loops back to action 402, where the process flowrepeats. If, however, the result of the determination is positive (i.e.,that the test failed), then the process flow advances to action 424.

At action 424 the microcontroller 104 sets the status to hardwarefailure and potentially takes further action. For example, upon a statusof hardware failure (and/or upon a change in status from something elseto such), the microcontroller 104 may operate to display the status atlocation 306, by highlighting the label “HW Failure”. Alternatively oradditionally, the microcontroller 104 may operate to store such statusinto an event log, which may include one or more of the test currentmeasurement, the threshold(s), the time, the date, the duration of thecondition, etc. Additionally or alternatively, the microcontroller 104may operate to send a notification of the status (or change in status)to one or more destinations in a manner similar to that discussed above.After completing the processes of action 424, the process flow loopsback to action 402, where the process flow repeats.

Reference is now made to FIG. 5, which illustrates a more detailedschematic diagram of a circuit suitable for implementing the systemsdiscussed above, and/or other embodiments disclosed or described herein.In many ways the system 100A is similar to the system 100 discussedabove. A more specific (and/or alternative) example for implementing theleakage current sensor is illustrated in FIG. 5.

In the system 100A, the source of AC power and the load circuit 140 areon a primary side of an electrical isolation boundary, and themicrocontroller 104 is on a secondary side of the electrical isolationboundary. Maintaining the isolation boundary is desirable, and oftenrequired by law or specification, to protect operators using/maintainingthe load 140 and/or the microcontroller 104 from shock or other hazardsassociated with the AC source.

The leakage current sensor 202 includes a first circuit 202A on theprimary side of the electrical isolation boundary, and a second circuit202B on the secondary side of the electrical isolation boundary.Additionally, the leakage current sensor 202 includes one or more thirdcircuits that inter-couple the first and second circuits 202A, 202B ofthe leakage current sensor 202, yet maintain the electrical isolationboundary therebetween. In this example, the one or more third circuitsinclude the differential current transformer T100 and an isolatorcircuit 204. The differential current transformer T100 maintainsisolation because there is no direct connection from the secondarywinding 120 to the primary winding (the phase and neutral wires L, N).The transformer includes one or more turns of the phase and neutralwires L, N in a bifilar configuration, and a plurality of turns onsecondary winding 120 to establish a 1000:1 turns ratio.

Similarly, the isolator circuit 204 maintains isolation because there isno direct connection from the primary side to the secondary side. Inparticular, the isolator circuit 204 includes a primary side circuit208, and a secondary side circuit 206. The primary side circuit 208operates to induce an unbalanced flux in the transformer core thatproduces a current in the secondary winding 120 of the differentialcurrent transformer T100. The unbalanced flux is in response to anintermediate control signal (within the isolator circuit 204) crossingthe electrical isolation boundary from the secondary side thereof. Inthis example, the isolator circuit 204 includes a light emitting devicein the secondary side circuit 206, which produces light when activatedby the microcontroller 104 (not shown). The intermediate control signal(the light) crosses the electrical isolation boundary from the secondaryside to the primary side thereof. The isolator circuit 204 also includesa light sensitive switch circuit (in this example a TRIAC) in theprimary side circuit 208 that turns on and off in response to the lightof the intermediate control signal. The light emitting device 206 is theLED cathode of the TRIAC, and is connected to the GPIO pin of themicrocontroller 104. When the microcontroller 104 sets the GPIO pin tothe OFF state, a current flows through R201 and the LED 206 of theTRIAC, which causes the light sensitive switch 208 of the TRIAC to turnon.

The leakage current sensor 202 includes an additional test winding onthe transformer T100 that produces the unbalanced flux in thetransformer core in response to the light sensitive switch circuit 208turning on. In particular, the light sensitive switch circuit 208, thetest winding and a resistance R210 (in this example implemented using aplurality of resistors) are coupled in series between the phase wire Land the neutral wire N, such that the unbalanced flux is produced inresponse to the intermediate control signal (the light) turning on theswitch circuit 208. The current flowing through the test winding inducesthe unbalanced flux, thereby generating the induced current in thesecondary winding 120 of the transformer T100. The net result of turningon the switch 208 of the TRIAC is to cause a self-test (faux) leakagecurrent equal to V×T/R, where V is the primary AC voltage, T is thenumber of turns (e.g., about one or two) of the test winding about thecore, and R is the resistance. For example, if V=230, T=1, and R=3990, aleakage current of 5.76 mA will be generated.

The secondary side circuit 206 includes a burden resistor R205, whichmay be about 100 ohms. The voltage produced across the burden resistorR205 is equal to 0.1× the actual leakage current, which adheres to thefollowing equation:V=ILEAK/N×R205,where ILEAK is the leakage current, N is transformer turns ratio, andR205 is value of the burden resistor in ohms.

The differential voltage, V, across R205 is converted to a single-endedvoltage output by operational amplifier U200. The operational amplifierU200 is configured as a differential amplifier with a gain of 100 (asset by the ratios R208/R206 and R209/R207). Assuming that resistorsR208=R209=100K, and R206=R207=1K, this results in an output voltage,VOUT=10×ILEAK, which is input into the microcontroller 104, preferablythe ADC input thereof. To eliminate or reduce high frequency noisepickup, capacitors C201 and C202 (e.g., about 1.5 nF) are included toform a first order low pass filter with a cut-off frequency of about1000 Hz. The operational amplifier U202 is configured as a buffer andprovides a bias point for U200. By way of example, the components of theoperational amplifier U202 may be established to produce a bias point ofabout 1.65 volts. Assuming the example of a test leakage current of 5.76mA, a 57.6 mV signal will be produced at the output of U200.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

The invention claimed is:
 1. An apparatus, comprising: a leakage currentsensor configured to measure leakage current from a source of AC poweron a primary side of an electrical isolation boundary, which providespower to at least one load circuit on said primary side of saidelectrical isolation boundary; said leakage current sensor including (i)a first circuit on the primary side of the electrical isolationboundary; (ii) a second circuit on a secondary side of the electricalisolation boundary; and (iii) a third circuit that inter-couples thefirst and second circuits of the leakage current sensor whilemaintaining the electrical isolation boundary therebetween, including atleast (a) a differential current transformer having a primary windingformed by one or more equal numbers of turns of respective legs about atransformer core as such legs pass from the source of AC power to the atleast one load circuit, and a secondary winding formed by a plurality ofturns of wire about the transformer core; and (b) an isolator circuithaving: (1) a primary side circuit operating to induce an unbalancedflux in the transformer core that produces a current in the secondarywinding of the differential current transformer, the unbalanced fluxbeing in response to an intermediate control signal crossing theelectrical isolation boundary from the secondary side thereof; and (2) asecondary side circuit operating to produce the intermediate controlsignal in response to a test control signal produced by a controlcircuit, said secondary side circuit including a switch circuit thatturns on and off in response to the intermediate control signal; a testwinding of one or more turns on the differential current transformer;and a resistance; wherein the switch circuit, test winding, andresistance are coupled in series between the legs such that theunbalanced flux is produced in response to the intermediate controlsignal turning the switch circuit on; and a control circuit on asecondary side of the electrical isolation boundary configured to: (i)receive measured leakage current values from the leakage current sensorat least at some times other than during a self test of the leakagecurrent sensor, (ii) compare the measured leakage current values againsta plurality of predetermined thresholds at least at some times otherthan during a self test of the leakage current sensor; (iii) provide astatus output indicating the status of leakage current with respect tosaid plurality of thresholds; and (iv) perform a self test to determinewhether the leakage current sensor is operative by providing said testcontrol signal.
 2. The apparatus of claim 1, wherein: the controlcircuit is programmable by an operator in order to set at least one ofthe predetermined thresholds; the predetermined thresholds include atleast one of: (i) a first threshold level indicating a measure leakagecurrent that is excessive; and (ii) a second threshold level, higherthan the first threshold level; and the control circuit operates tostore and retrieve results of the comparisons of the measured leakagecurrent values against the predetermined thresholds.
 3. The apparatus ofclaim 2, wherein at least one of: the control circuit is programmable byan operator in order to set at least one of: (i) periodic self test timeintervals; (ii) one or more dates of the self tests, and (iii) one ormore times for the self tests; and the control circuit operates to storeand retrieve results of one or more of the self tests for validation ofthe leakage current sensor.
 4. The apparatus of claim 3, wherein thecontrol circuit operates to store into an event log at least one of: theresults of one or more of the self tests; and the comparisons of themeasured leakage current values against the one or more predeterminedthresholds.
 5. The apparatus of claim 4, wherein the control circuitoperates to store the status indicators into the event log, including atleast one of: whether the measured leakage current values are at normallevels, because the comparison indicates that the measured leakagecurrent values are below the first threshold level; whether the measuredleakage current values are at warning levels, because the comparisonindicates that the measured leakage current values are above the firstthreshold level but below the second threshold level; whether themeasured leakage current values are at critical levels, because thecomparison indicates that the measured leakage current values are abovethe second threshold level; whether a self test is in progress; whetherresults of one or more self tests indicate that the leakage currentsensor is non-operative; and whether results of one or more self testsindicate that the leakage current sensor is operative.
 6. The apparatusof claim 3, further comprising a display operatively coupled to thecontrol circuit, wherein the display shows a visual indication of atleast one of: the measured leakage current values; the first thresholdlevel; the second threshold level; the periodic self test timeintervals; the one or more dates of the self tests; and the one or moretimes for the self tests.
 7. The apparatus of claim 3, furthercomprising a display operatively coupled to the control circuit, whereinthe display shows a visual indication of the status, the statusincluding at least one of: whether the measured leakage current valuesare at normal levels, because the comparison indicates that the measuredleakage current values are below the first threshold level; whether themeasured leakage current values are at warning levels, because thecomparison indicates that the measured leakage current values are abovethe first threshold level but below the second threshold level; whetherthe measured leakage current values are at critical levels, because thecomparison indicates that the measured leakage current values are abovethe second threshold level; whether a self test is in progress; whetherresults of one or more self tests indicate that the leakage currentsensor is non-operative; and whether results of one or more self testsindicate that the leakage current sensor is operative.
 8. The apparatusof claim 3, further comprising a message transmission system coupled tothe control circuit and operating to transmit one or more messagesindicating at least one of: the results of one or more of the selftests; and the comparisons of the measured leakage current valuesagainst the one or more predetermined thresholds.
 9. The apparatus ofclaim 8, wherein the message transmission system is operable to send theone or more messages in at least one of: (i) a simple network managementprotocol, (ii) a packet switched network protocol, (iii) an electronicmail protocol, (iv) an instant messaging protocol, and (v) a telephoneprotocol.
 10. The apparatus of claim 9, wherein the message transmissionsystem is programmable by an operator to select the messaging protocol.11. The apparatus of claim 10, further comprising a display operativelycoupled to the control circuit, wherein the display shows a visualindication of the selected messaging protocol.
 12. The apparatus ofclaim 9, wherein the status indicators include at least one of: whetherthe measured leakage current values are at normal levels, because thecomparison indicates that the measured leakage current values are belowthe first threshold level; whether the measured leakage current valuesare at warning levels, because the comparison indicates that themeasured leakage current values are above the first threshold level butbelow the second threshold level; whether the measured leakage currentvalues are at critical levels, because the comparison indicates that themeasured leakage current values are above the second threshold level;whether a self test is in progress; whether results of one or more selftests indicate that the leakage current sensor is non-operative; andwhether results of one or more self tests indicate that the leakagecurrent sensor is operative.
 13. A method, comprising: measuring leakagecurrent from a source of AC power, which provides power to at least oneload circuit, said source of AC power and said load circuit being on aprimary side of an electrical isolation boundary; comparing the measuredleakage current values against a plurality of predetermined thresholds;providing a status output indicating the status of leakage current withrespect to said plurality of thresholds; and automatically performingself tests to determine whether the leakage current measuring andcomparing operations are operative, wherein comparing the measuredleakage current values and providing a status output are performed on asecondary side of said electrical isolation boundary electricallyisolated from the primary side, and performing self tests is triggeredby a test control signal from said secondary side of said isolationboundary.
 14. The method of claim 13, further comprising permitting anoperator set at least one of: the plurality of predetermined thresholds;and timing at which the self tests are performed.
 15. The method ofclaim 14, wherein the predetermined thresholds include at least one of:(i) a first threshold level indicating a measure leakage current that ishigher than desired; and (ii) a second threshold level, higher than thefirst threshold level; and the timing at which the self tests areperformed include at least one of: (i) periodic self test timeintervals; (ii) one or more dates of the self tests, and (iii) one ormore times for the self tests.